Deadlifts, shrugs. The neck muscles contract in just about every lift you do, so as long as you keep lifting, you'll have a big neck that makes buying dress shirts a pain in the ass in no time.
Deadlifts are #1.
Actually all heavy compound movements thicken my neck - cheat culs, overhead press, squat, bench press, BB rows, ect. I never train to isolate my neck.